Sighting apparatuses, in particular telescopes, are required for many applications, in particular for geodetic, constructional and military applications as well, for example for direct visual observation through an eyepiece and/or for image recording by a camera. Within the scope of geodesy, the coupling of a pure observation with a distance measurement to a sighted target object is of particular importance here, for the purposes of which, in addition to the optical system components for a telescope beam path, an integration of optical assemblies for coupling and decoupling transmission rays or distance measuring rays is required in a corresponding geodetic instrument. A required high measurement accuracy of the distance measurement, in particular, demands very stable positions and angles of the involved beam paths and assigned assemblies.
The optical setup of telescopes is, in particular, determined by the function of the telescope, for example in view of light intensity, imaging quality and focusing and magnification mechanisms. In contrast to optical measurement instruments, such as e.g. an electro-optical rangefinder with an optical receiver for receiving beams, the beams in the telescope are received e.g. by the human eye, for the purposes of which imaging with a corresponding very high quality is necessary. A challenge for engineers in the field of optics lies in the production of telescopes with a short installation length and, nevertheless, a generic high imaging quality. In order to achieve this imaging quality, it is necessary to correct optical aberrations, such as spherical and chromatic aberration, coma and distortion. These corrections are performed by means of optical components, wherein parameters such as the curvature of lenses, the number, the material with the associated optical properties and the arrangement of the corrective components, and also the highly precise manufacturing of same and the exact alignment in the telescope contribute to the image quality.
In a telescope, the demanded high imaging quality requires the generation of a diffraction-limited image in terms of the resolution thereof in the image plane of the telescope. In particular, the image circle radius in the image plane—that is the light spot of the receiving beam from a target object in the image plane caused by the aforementioned optical aberrations—as a lower limit should only be determined by the diffraction limit and only be a fraction of a micrometer. In order to be able to achieve this, corrections for limiting the (overall) aberrations to less than a quarter of the wavelength λ of the visible light, i.e. to approximately 100 nm to 200 nm, are required.
Here, the correction of the chromatic aberrations is always a challenge. It is well known that the dependence of the refractive index of a medium, thus, in particular, also of a refractively acting lens, on the wavelength causes a mutual offset of the image plane/“in-focus plane” for different wavelengths. Achromatic lenses (achromats) were developed as a conventional remedy for this phenomenon. These achromats, which are most widespread as an “achromatic doublet”, are characterized as a composition of components with different dispersion, i.e. with a different refractive index for different wavelengths. Here, use is most frequently made of an ensemble of elements made of flint glass and crown glass.
Using achromats, it is possible to realize a chromatically at least approximately aberration-free image for two wavelengths. However, the images for the other wavelengths outside of the chromatically corrected range still lie in front of or behind the in-focus plane; this residual error is also referred to as “secondary spectrum”. The chromatic aberration can be further reduced by using a combination of more than two lenses with different materials.
In the description, a distinction is made between “achromats” and “apochromats”; this denotes the type of correction, namely for two or three correctly focused wavelengths, but not the extent of the defocusing of the other wavelengths. The mode of action of both achromat and apochromat can therefore be improved, even outside the specific correction region, by the use of glasses with a dispersion that is as low as possible instead of regular glasses with typically relatively high dispersion. Glasses containing fluorite (CaF2) have a particularly low dispersion, and so two lenses made of this material, combined with one another, can already cause a very extensive correction.
The difference in the Abbe numbers for different wavelengths is also referred to as “partial dispersion” P. In addition to the refractive index n and the Abbe number ν, the relative partial dispersion P is also important as a quality criterion for visual systems. By way of example, the relative partial dispersion Pg,F relates to the two wavelengths g=435.8 nm and F=486.1 nm. Normal glasses are characterized by a relative partial dispersion Pg,F=ag,F+bg,F·νd, which relation describes the so-called normal line. The constants ag,F and bg,F are usually set to ag,F=1.7241 and bg,F=−0.008382, and νd denotes the Abbe number in relation to the wavelength d=587.6 nm. By way of example, these relations are depicted and described in Naumann/Schröder: “Bauelemente der Optik [Optical modules]”, Chapter 3.3.2 “Optische Gläser [Optical glasses]” (Carl Hanser Verlag publishers, 6th edition).
The optimization of a chromatic correction can be described mathematically. To this end, for a doublet consisting of two thin lenses which are in contact with one another, the Abbe number of the materials is used for determining the correct focal length of the lenses. The following demand is raised:f1·ν1+f2·ν2=0
Here, f1 and f2 denote the focal lengths of the two lenses for the Fraunhofer D-line (λ=589.2 nm) and ν1 and ν2 are the associated Abbe numbers of the materials of the first and second lens. Since the Abbe numbers of the lenses are positive, one of the focal lengths must be negative, i.e. the associated lens must be divergent, in order to fulfill the condition.
The principle of the chromatic aberration will be illustrated and explained in more detail in conjunction with the figures of this patent application.
For imaging systems, such as telescopes, there is a demand for an in-focus image over the whole visual spectrum where possible, i.e. there is a demand for the reduction or elimination of the secondary spectrum. What is typically demanded is that the longitudinal chromatic aberration (secondary spectrum) is less than 0.2% of the focal length of an objective.
A multiplicity of optical special glasses which, however, are relatively expensive in part are known for correcting optical aberrations for visual systems. Special glasses with a relative partial dispersion which deviates—possibly strongly—from the relative partial dispersion of the normal glasses are particularly suitable for a chromatic correction (reduction in the chromatic aberration) over a broad spectral range, i.e. for reducing or lifting the secondary spectrum.
Conventional telescopes with refractive optical elements, predominantly glass lenses, often have a number of disadvantages, particularly also as a result of the aforementioned various corrections which are required for generating a high-quality image of a target object. By way of example, these relate to a large installation size of the optics, a multiplicity of discrete individual elements and a high overall weight and a difficult and complicated adjustment of the individual elements in the beam path which, together, lead to high production costs.
In order to overcome these disadvantages, various approaches were followed in recent years. These related, firstly, to a replacement of glass elements with plastic elements. This was rendered possible by the fact that the optical quality of plastic products can in the meantime largely approach the optical quality of glass products. This replacement enables a first weight reduction and also a reduction in the individual element costs, in particular if a production by forming or injection molding is possible. The system costs can be reduced, in particular, if the use of integral components for meeting a plurality of functions is possible; this also reduces the number of necessary individual adjustment steps.
Further advantages emerge, secondly, by a use of diffractive optical elements, e.g. Fresnel lenses, as a result of which the strength of the directional influence/deflection of a light ray no longer depends on the optical path length in the optically passed-through optical element, but rather depends on the type and geometric distribution of the diffractive structures of a diffractive optical element, as a result of which a further reduction in the volume and hence the mass and weight of the individual elements is achieved. Setup and function and Fresnel lens will likewise still be illustrated and explained in more detail in conjunction with the figures.
By means of diffractive optical structures for the replacement of glass lenses it is possible to replace systems made of a plurality of individual lenses by a single element. In the process, both spherical aberrations and chromatic aberrations can be corrected, with diffractive optical elements being particularly well-suited for this. In particular, the first diffractive order (first order of diffraction) of a diffractive optical element is used for chromatic correction. To this end, the Abbe number in the visible spectrum is typically between −3.5 and −3.3 (dependent on the respectively used material), and so it is negative in contrast to optical elements (e.g. made of glass or plastic) with a refractive effect. Conventional glasses have an Abbe number of between 20 and 80. As a consequence, a diffractive lens with only a very weak effect is required for the chromatic correction of a refractive lens. Corrections with higher orders of diffraction are not found to be advantageous since the diffractive structure loses its diffractive character and exhibits ever more refractive properties.
The patent document DE 199 41 638 C1 discloses a geodetic instrument with a telescope and a laser arrangement for emitting measurement radiation, in which a diffractive optical element is used in an excitation beam path for the emitted laser light. Here, the diffractive optical element is embodied as a transparent plate provided with a structure hologram, which transparent plate is arranged upstream or downstream of a deflection element in the excitation beam path and serves to diffract the collimated excitation ray in a diverging concentric manner to a circular beam.
U.S. Pat. No. 6,501,541 B2 discloses an electronic distance measurement instrument with a sighting telescope and, in particular, a diffractive optical element, embodied as a Fresnel mirror or a diffractive grating, in a reception beam path of the instrument for a wavelength-selective beam deflection of the light from the target object in the observation beam path of the telescope. Hence, the use of the diffractive optical element in this application does not result in a basic reduction of weight or a reduction in installation size/component number of the instrument since the function used here can also be fulfilled by conventional wavelength-selective and ray-deflecting elements such as e.g. prisms, as is also confirmed in the description of this document.
U.S. Pat. No. 6,587,244 B1 relates to an optical communication system for a geodetic instrument, for example a theodolite. The invention underlying this patent document serves to address the problem of enabling a satisfactory function of the instrument both for short and long distances between the target object and instrument, without to this end having to lengthen the required detection path in the instrument. The optical arrangement disclosed in this patent comprises a diffractive optical element, in particular a Fresnel lens or a diffractive grating, upstream of a photodetector in a reception beam path, but no imaging optical system assigned to this reception beam path. The disclosed diffractive optical element transmits parallel light portions of a communication light without influencing the latter, in conjunction with a converging function at the zero and at the first order of diffraction for the purposes of guiding said light components to the photodetector.
WO 2010/097346 A1 discloses a geodetic instrument with a telescope. In order to bring about an effect of in-focus imaging on the receiving surface of an employed detector of all points of a target object plane lying within a registration region of the associated optical arrangement, which effect is wanted according to the invention, the use of an imaging objective having a field curvature, in particular along the horizontal axis thereof, or the use of a diffractive structure which generates an angle of incidence-dependent variation in the focal length are proposed as alternatives to one another.
In the optical systems of the aforementioned arrangements from the prior art, diffractive optical elements are in each case used only for fulfilling a single functionality. The results achieved in the process do not appear ideal with absolute certainty, for example in relation to the optical imaging quality/chromatic correction.